Sensorless motor control

ABSTRACT

An electric motor control system includes a power inverter and control circuitry configured to control the power inverter either according to a target voltage in a voltage-based control mode or according to a target current in a current-based control mode. A controller is operable to switch operation of the control circuitry between the voltage-based control mode and the current-based control mode. The controller may be configured to operate the control circuitry in the current-based control mode at lower motor operating speeds where stator current margin is of greater significance, and to operate the control circuitry in the voltage-based control mode at higher motor operating speeds where stator voltage margin is of greater significance.

FIELD

Embodiments of the present invention relate to systems and methods forcontrolling electric motors.

BACKGROUND

Electric motors commonly include a stationary component called a statorand a rotating component called a rotor. The rotor rotates within (oraround) the stator when the motor is energized with a driving waveform.Induction motors, sometimes referred to as asynchronous motors, are atype of electric motor wherein power is supplied to the rotor by meansof electromagnetic induction rather than by means of direct electricalconnections to the rotor.

As with synchronous motors, the driving waveform supplied to aninduction motor's stator creates a magnetic field that rotates in timewith the AC oscillations of the driving waveform. The induction motor'srotor rotates at a slower speed than the stator field. This differencein rotational speed, also referred to as “slip,” “slip frequency,” or“slip speed,” results in a changing magnetic flux in the rotor windingsthat induces currents in the rotor windings. The induced current in turngenerates magnetic fields in the rotor windings that oppose the rotatingmagnetic field created by the stator, thereby inducing rotationalmovement in the rotor. The rotor accelerates until the magnitude ofinduced rotor current and rotor torque balances the applied load. Sincerotation at synchronous speed would result in no induced rotor current,an induction motor always operates at less than synchronous speed duringnormal forward operation.

When the frequency of the driving waveform falls below the rotorfrequency, such as where the programmed speed of the motor is reduced bya motor controller during operation, the rotating magnetic field createdby the stator induces rotational pressure on the rotor that opposes therotor's movement and reduces motor speed. In this braking mode ofoperation, the inertia of the rotor and applied load induces voltage inthe stator that may energize external motor components, such as a DC bussupplying power to the motor.

A known method of controlling electric motors is field oriented controlor “FOC.” Field oriented control involves controlling a motor usingthree motor input variables including voltage magnitude, voltage angleand frequency. Because stator current is closely related to outputtorque and other operating characteristics of the motor, it is desirableto use field oriented control methods to operate the motor according toa target stator current. At high motor speeds, however, manipulating thestator current using field oriented control becomes less effectivebecause the motor operates at or near a maximum voltage level,eliminating or reducing one of the input variables (voltage magnitude).

The above section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

Embodiments of the present invention solve the above described problemsby providing improved systems for controlling electric motors that arecapable of operating in different operating modes according to theoperating speed of the motor.

A motor control system constructed in accordance with an embodiment ofthe present invention comprises a power inverter, control circuitryconfigured to control the power inverter either according to a targetvoltage in a voltage-based control mode or according to a target currentin a current-based control mode, and a controller for switchingoperation of the control circuitry between the voltage-based controlmode and the current-based control mode.

In a related embodiment, the control circuitry includes a plurality ofregulators for generating and/or adjusting motor control signals. Afirst regulator receives a differential speed signal and generates atorque producing component i_(qref) of a reference current signal basedon the differential speed signal, the differential speed signalrepresenting a difference between a reference motor speed and anestimated motor speed. A second regulator receives a reference speedsignal and generates a flux producing component i_(dref) of thereference current signal.

A third regulator receives a differential i_(q) signal and generates atorque producing component V_(q) of a reference voltage signal, thedifferential i_(q) signal representing a difference between i_(qref) andan estimated actual torque producing component i_(q) of a motor drivingwaveform. A fourth regulator receives a differential i_(d) signal andgenerates a flux producing component V_(d) of the reference voltagesignal based on the differential i_(d) signal, the differential i_(d)signal representing a difference between i_(dref) and an estimatedactual flux producing component i_(d) of the motor driving waveform.

In another embodiment of the invention, a method of operating a motorcontrol system comprises generating a first motor drive signal accordingto a target current if a reference motor speed is within a first speedrange and generating a second motor drive signal according to a targetvoltage if the reference motor speed is within a second speed range. Themethod further comprises automatically switching operation of the motorcontrol system between generating the first motor drive signal andgenerating the second motor drive signal as the target speed changesbetween the first speed range and the second speed range.

This summary is provided to introduce a selection of concepts in asimplified form that are further described in the detailed descriptionbelow. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Other aspectsand advantages of the present invention will be apparent from thefollowing detailed description of the embodiments and the accompanyingdrawing figures.

DRAWINGS

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a block diagram of a motor system constructed in accordancewith embodiments of the invention.

FIG. 2 is a schematic representation of control circuitry for use withthe motor system of FIG. 1.

FIG. 3 is a schematic representation of an alternative embodiment of thecontrol circuitry of FIG. 2.

FIG. 4 is a graphical representation of certain operating andperformance characteristics of the motor of FIG. 1.

The drawing figures do not limit the present invention to the specificembodiments disclosed and described herein. The drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the invention.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying drawings. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thoseskilled in the art to practice the invention. Other embodiments can beutilized and changes can be made without departing from the scope of theclaims. The following detailed description is, therefore, not to betaken in a limiting sense. The scope of the present invention is definedonly by the appended claims, along with the full scope of equivalents towhich such claims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features being referred to areincluded in at least one embodiment of the technology. Separatereferences to “one embodiment”, “an embodiment”, or “embodiments” inthis description do not necessarily refer to the same embodiment and arealso not mutually exclusive unless so stated and/or except as will bereadily apparent to those skilled in the art from the description. Forexample, a feature, structure, act, etcetera described in one embodimentmay also be included in other embodiments, but is not necessarilyincluded. Thus, the present technology can include a variety ofcombinations and/or integrations of the embodiments described herein.

Turning now to the drawing figures, and initially FIG. 1, an electricmotor system 10 constructed in accordance with embodiments of theinvention is illustrated. The motor system 10 broadly includes anelectric motor 12, a power source 14, and a motor control system 16 forreceiving power from the power source 14 and generating a drivingwaveform to power the motor 12 according to a reference motor speed(ω_(ref)) communicated to the control system via a reference speed input18. The reference motor speed corresponds to a desired or targetoperating motor output speed and may be generated by an external motorcontrol mechanism (not illustrated) configured to operate the motor 12according to a particular implementation of the invention. The motorcontrol system 16 includes control circuitry 20 configured to receiveinput power from the power source 14, receive a reference motor speedsignal from the reference speed input 18, create the driving waveform,and communicate the driving waveform to the motor 12. A controller 22manages certain aspects of the operation of the control circuitry 20,including switching between a voltage-based control mode and acurrent-based control mode, as explained below.

The power source 14 may be a conventional alternating-current powersource, such as a standard 115V or 230V source available in residentialand commercial buildings via standard electrical outlets. The motorsystem 10 may be part of an appliance drive system, such as a washingmachine or a clothes dryer, that includes additional electrical andmechanical components not illustrated or described herein.

The motor 12 may be an electric induction motor and, in one exemplaryembodiment, is a three-phase, four-pole AC induction motor rated tooperate at a maximum voltage of between 190 and 200 volts and a maximumcurrent of between 4 and 6 amps. The motor 12 may be configured tooperate within a normal operating range of, for example, between 0 and3,000 rpm, and may be capable of operating at speeds well beyond thenormal operation range, such as up to 17,000 or 18,000 rpm. Motoroperation beyond the normal operating range may be characterized byprogressively diminishing performance characteristics, such as decreasedtorque, as explained below.

Various operating and performance characteristics of the motor 12,including stator voltage (V_(s)), maximum operating current and outputtorque, are illustrated in the graph of FIG. 4 across the operatingspeed range of the motor 12. As shown in FIG. 4, motor performancecharacteristics differ according to whether the motor is operating in anormal operating region or a field weakening region. The normaloperating region corresponds to a motor output speed of zero up to anominal operating speed 24, which may be, for example, 3,000 or 4,000rpm. The field weakening operating region corresponds to the nominalspeed 24 up to a maximum motor speed 26, which may be, for example,about 17,000 or 18,000 rpm. When operating in the normal operatingregion, the speed of the motor 12 generally follows a linear pattern andis proportional to the stator voltage (V_(s)), while torque is generallyconstant. In the field weakening region, the stator voltage remains ator near a maximum operating voltage and the torque diminishes as themotor output speed increases. The maximum operating current is definedby limitations of the motor 12, the control circuitry 20, or both. Whilethe maximum operating current generally remains constant across allmotor operating speeds, the motor stator current (i_(s)) will typicallyfluctuate with changing operational parameters, such as changes in motorspeed and load torque. The total stator current (i_(s)) includes atorque producing component (i_(sq)) and a flux producing component(i_(sd)), whose relationship is defined by the following equation:

i _(s)=√{square root over ((i _(sd))²+(i _(sq))²)}{square root over ((i_(sd))²+(i _(sq))²)}

For purposes of the present teachings, the motor 12 generally operatesin three different output speed ranges including low speed, medium speedand high speed. The low speed range generally corresponds to the normaloperating region, the medium speed range generally corresponds to alower portion of the field weakening region, and the high speed rangegeneral corresponds to an upper portion of the field weakening region,as indicated in the graph of FIG. 4. The motor speed corresponding tothe boundary between the medium and high speed ranges may be defined asa particular motor speed, such as 8,000 rpm or 10,000 rpm, or as apercentage of a maximum motor speed, such as 50%, 60% or 70% of themaximum speed 26. Alternatively, the boundary may be defined in terms ofthe stator current, such as a particular value of i_(sd) or i_(sq) or adifference between i_(sd) and i_(sq). Further yet, the boundary betweenthe medium and high speed ranges may be defined in terms of the torque,such as a particular torque value or percentage of maximum torque. Theboundary between the medium and high speed ranges is relevant in thatmay be used by the controller as a threshold speed for switching betweenthe voltage-based control mode and the current-based control mode, asexplained below. In some embodiments, the boundary between the mediumand high speed ranges is defined as a rotor speed at which the statorvoltage amplitude is at or near the inverter voltage margin. When themotor is operating at or near the inverter voltage margin, fluctuationsin the inverter voltage margin due to, for example, variations in the DCbus voltage make current-based motor control impractical.

The controller 22 includes digital logic configured to interact with andcontrol certain components of the control circuitry 20. The digitallogic may include discrete logic components, programmable logic devicesand/or general purpose computer processors such as microcontrollers ormicroprocessors. In some embodiments of the invention, the controller 22is or includes a computer processor configured to execute a computerprogram for directing the control circuitry 20 to operate in either thecurrent-based control mode in the low and medium speed ranges or in thevoltage-based control mode in the high speed range.

Thus, aspects of the invention may be implemented with one or morecomputer programs stored in or on computer-readable media residing in oraccessible by the controller 22. The computer program comprises a seriesof executable instructions for implementing logical functions in thecontroller 22. The computer program can be embodied in anynon-transitory computer-readable medium for use by or in connection withthe controller 22 wherein the controller fetches instructions from thecomputer-readable medium and executes the instructions. As used herein,a “computer-readable medium” can be any non-transitory means that canstore the program for use by or in connection with the controller. Thecomputer-readable medium can be, for example, but is not limited to, anelectronic, magnetic, optical, electro-magnetic, infrared, orsemi-conductor system, apparatus, or device. More specific, although notinclusive, examples of the computer-readable medium would include thefollowing: an electrical connection having one or more wires, a randomaccess memory (RAM), a read-only memory (ROM), and an erasable,programmable, read-only memory (EPROM or Flash memory).

Turning now to FIG. 2, certain components of the control circuitry 20are illustrated in greater detail. A rectifier 30 converts AC powerreceived from the power source 14 to DC power that energizes a DC bus32. The rectifier 30 may include virtually any type of rectifiercircuit, but in various preferred embodiments is a simplenon-regenerative rectifier, such as a diode bridge rectifier. A signalshunt 34 and voltage sensor 35 are coupled with the DC bus 32 and usedto detect voltage and/or current levels on the DC bus 32 for use infeedback loops, as explained below in greater detail. The signal shunt34 may be, for example, a resister, while the voltage sensor 35 may be avoltage divider circuit. One or more capacitors 36 are also coupled tothe DC bus 32 to minimize voltage ripple on the bus 32 and to storeregenerative energy in the bus 32 during braking.

Inverter circuitry 38 includes pulse width modulation (“PWM”) controllogic 40 and a three-phase inverter 42 that converts DC power from theDC bus 32 to driving waveforms that are communicated to the motor 12.The PWM control logic 40 receives two input signals (V_(αref) andV_(βref)) representing two-phase stationary reference frame voltagevalues V_(α) and V_(β), and converts the input signals to invertercontrol signals that drive the inverter to convert power from the DC bus32 to the driving waveforms applied to the motor stator windings. Thevoltage magnitude, voltage angle and frequency of the driving electricwaveforms are manipulated according to the reference motor speed ω_(ref)to control operation of the motor 12. When the motor 12 is operating ata constant speed or accelerating, it has a positive slip frequency andgenerally consumes energy. When the motor 12 is decelerating (braking),it has a negative slip frequency and generates energy that is dissipatedin the motor 12 (as rotor and stator losses) and that energizes theinverter 42 and the DC bus 32, thereby increasing the voltage in thosecomponents beyond the voltage supplied by the rectifier 30. Theincreased voltage on the DC bus 32 may contribute to braking by furtherincreasing stator voltages and losses in the motor 12, as explainedbelow in greater detail.

When the switch 28 is in a first position (represented by the brokenlines in FIG. 2), the control circuitry 20 operates in the current-basedcontrol mode according to principles of indirect field oriented control(IFOC). In the current-based control mode, the control circuitry 20drives operation of the power inverter 42 according to a target statorcurrent. The IFOC motor control method involves controlling AC motoroperation by means of three motor input variables: voltage magnitude,voltage angle and signal frequency. These variables are manipulatedaccording to the reference speed ω_(ref) via two control/feedback loopsthat independently control the speed (frequency) and strength of therotating magnetic field induced in the motor's stator.

The reference motor speed (ω_(ref)) signal is received at the left sideof the circuit diagram in FIG. 1. The ω_(ref) signal may be receivedfrom an external motor controller, such as an appliance motor controllerconfigured to control the speed and operation of the motor according tooperational parameters of the particular appliance. An estimated rotorspeed, ω_(r), is subtracted from the reference speed and the differenceis communicated to a first proportional-integral controller 44configured to generate a torque-producing component i_(ref) of a targetstator current. The estimated motor speed (ω_(r)) is also communicatedto a field weakening controller 46 configured to generate aflux-producing component i_(dref) of the stator current. The fieldweakening controller 46 decreases the value of i_(dref) when thereference motor speed is in the field weakening region of motoroperation.

The i_(qref) signal is communicated from the first proportional-integralcontroller 44 to a DC voltage limiter circuit 48 that is configured tolimit the maximum negative value of i_(qref) based on the voltagelimitations of the motor 12, inverter 42 and/or capacitor 36. The valueof i_(qref) is negative during motor braking when the reference motorspeed is less than the actual rotor speed. When braking, regenerativeenergy from the motor 12 increases the voltage in the inverter 42 andthe DC bus capacitor 36, as explained above. The voltage limiter circuit48 prevents the negative value of i_(qref) from exceeding a levelcorresponding to the maximum voltage of the motor, inverter and/orcapacitor. The voltage limiter circuit 48 receives a first signalV_(dcmax) representing a maximum voltage level, a second signal V_(dc)representing an estimated or measured voltage on the DC bus 32, andsubtracts the value of V_(dc) from V_(dcmax). An integrator controller50 integrates the difference and communicates the result, −i_(qrefmax),to a first current limiter component 52 that limits i_(qref) to thevalue of −i_(qrefmax).

A current limiter component 54 receives both i_(qref) and i_(dref)signals and imposes a limit on the signals corresponding to a maximumstator current as defined by a maximum rating of the motor 12 and/or theinverter 42. Thus, while the voltage limiter circuit 48 imposes amaximum negative value of i_(qref) during braking to regulateregenerative voltage, the current limiter component 54 imposes a maximumpositive value of i_(qref) and i_(dref) during normal operation.Measured values of the torque-producing component i_(q) and theflux-producing component i_(d) of the actual stator current aresubtracted from i_(qref) and i_(dref), and the resulting difference iscommunicated to second 56 and third 58 proportional-integral controllerswhich calculate rotating two-phase voltage signals V_(qref) andV_(dref).

An inverse Park conversion circuit 60 converts the V_(qref) and V_(dref)signals to stationary two-phase signals V_(αref) and V_(βref) which arereceived by the PWM control logic 40 of the inverter circuitry 38. ThePWM control logic 40 generates a plurality of inverter control signalsbased on the stationary two-phase signals V_(αref) and V_(βref), asexplained above, and communicates the control signals to the inverter42.

In the current-based control mode, the control circuitry 20 usesfeedback loops to generate the motor control signals. The shunt 34 andsensor 35 detect current and/or voltage on the DC bus 32 and communicatethe information to a converter component 62 that generates a pluralityof signals representing measured values of the instantaneous statorcurrent and/or voltage. A Clarke conversion circuit 64 converts themeasured stator current and/or voltage values to two-phase, stationaryvalues i_(α) and i_(β), which are then converted to two-phase rotatingsignals i_(q) and i_(d) by a Park conversion circuit 66. The i_(q) andi_(d) signals are combined with the i_(qref) and i_(dref) signals, asexplained above. The Clarke conversion circuit 64 may also generatetwo-phase, stationary values V, and V_(p) that are used by a speedestimator 68 to estimate the rotor speed ω_(r).

The speed estimator 68 calculates an estimated rotor speed, ω_(r), basedon characteristics of the power communicated to the stator, such as theestimated values i_(α), i_(β), V_(α) and V_(β) generated by the Clarkeconversion circuit 64. The speed estimator 68 uses, for example, modelreference adaptive system (MRAS) principles of operation. The ω_(r)signal is combined with ω_(ref) to generate i_(qref), as explainedabove. The ω_(r) signal is also used by an output angle calculationcircuit 70 to generate an output angle signal θ_(dq) that is used by theClarke and inverse Clarke conversion circuits to translate between thestationary and rotating domains.

The output angle calculation circuit 70 includes a slip calculator 72that determines slip frequency, ω_(s), as a function of i_(qref) andi_(dref). Specifically, the slip frequency may be defined using thefollowing equation:

$\omega_{s} = {\frac{i_{qref}}{i_{dref}} \times k}$

where the value of k depends on the characteristics of the motor 12. Theslip frequency is combined with the estimated rotor frequency, ω_(r),and that combination is integrated using an integrator 74 to generatethe output angle signal θ_(dq).

As mentioned above, when operating in the current-based control mode thecontrol circuitry 20 uses two independent feedback loops to control thestrength and speed of the rotating magnetic field induced in the stator.A first feedback loop controls the strength of the rotating magneticfield and includes the Park 66 and Clarke 64 conversion circuits and theproportional-integral controllers 56, 58. A second feedback loopcontrols the speed of the rotating magnetic field and includes theClarke conversion circuit 64, the speed estimator 68, the firstproportional-integral controller 44, the field weakening controller 46and the output angle calculation circuit 70. Thus, there is some overlapbetween the two feedback loops.

When the switch 28 is in the second position (indicated by solid linesin FIG. 2) the control circuitry 20 operates in the voltage-basedcontrol mode and controls the motor 12 by manipulating the voltage andfrequency of the driving waveform rather than using the IFOC mode ofoperation employed in the current-based control mode. In the illustratedembodiment, V_(dref) is set to zero and V_(qref) is set to V_(rated),which may correspond to the maximum operating voltage. The maximumoperating voltage will typically be the voltage available on the DC bus32, but may also be related to the maximum voltage capacity of theinverter 42 and/or the motor 12.

In the voltage-based control mode, the control circuitry 20 operates theinverter 42 according to a target voltage (V_(rated)) rather than atarget current. This mode of operation may be desirable, for example,where the motor 12 is operating at very high speeds and stator voltageis at or near the maximum voltage. In such situations, the voltage-basedcontrol mode allows greater power to be delivered to the motor 12because the voltage margins associated the current-based control modeare eliminated.

Control circuitry 100 constructed according to another embodiment of theinvention is illustrated in FIG. 3. The control circuitry 100 isidentical to the control circuitry 20 illustrated in FIG. 2 anddescribed above, except that the control circuitry 100 includes brakemechanisms for regulating the braking power of the motor 12 when thecontrol circuitry 100 is operating in either the current-based controlmode or in the voltage-based control mode. Therefore, for conveniencesimilar components are assigned the same reference numerals and only thedifferences between the two embodiments will be discussed. Particularly,a current-based braking mechanism 102 is used when the circuitry 100 isoperating in the current-based control mode and a voltage-based brakingmechanism 104 is used when the circuitry 100 is operating in thevoltage-based control mode.

The current-based brake mechanism 102 determines a difference between achange in the reference speed (Aω_(ref)) and a change in the estimatedrotor speed (Δω_(r)). That difference is integrated using an integratorcontroller 106 to generate a differential speed signal Δi_(dref) that iscombined with the i_(dref) signal when braking in the current-basedcontrol mode. Thus, the current-based braking mechanism 102 augments thei_(dref) signal according to a difference between the reference motorspeed ω_(ref) and the estimated rotor speed ω_(r) thereby increasingmotor losses and causing the motor to slow at a quicker rate. As thedifference between the reference motor speed and the estimated motorspeed increases, so does the value of Δi_(dref) and i_(dref).

A first braking switch 108 is operable to selectively connect the outputof the current-based braking mechanism 102 to the i_(dref) signalcombination point. The controller 22 may be configured to actuate theswitch 108 to connect the current-based braking mechanism 102 to thei_(dref) signal combination point when the motor is in braking mode(i.e., when ω_(ref) is less than ω_(r)).

The voltage-based braking mechanism 104 augments the V_(rated) input toincrease motor losses during braking when the control circuitry 20 isoperating in the voltage-based control mode. The voltage-based brakingmechanism 104 is similar in form and function to the current-basedbraking mechanism 102, described above, such that the braking signalincreases with a difference between the reference speed and theestimated rotor speed. Augmenting the V_(rated) input causes the voltageapplied to the motor to increase as well, thus increasing motor lossesduring braking and causing the motor to slow at a quicker rate.Increasing the voltage applied to the motor also increases the level ofenergy generated in the inverter and on the DC bus by the motor, whichin turn increases the amount of voltage available to be applied to themotor, further increasing motor losses and braking power.

A second braking switch 110 is operable to selectively connect theoutput of the voltage-based braking mechanism 104 to the V_(rated)signal combination point. The controller 22 may be configured to actuatethe switch 110 to connect the voltage-based braking mechanism 104 to theV_(rated) signal combination point when the motor is in braking mode(i.e., when ω_(ref) is less than ω_(r)).

As explained above, the amount of energy generated in the inverter 42and the DC bus 32 by the motor 12 during braking will generally belimited by the rated capacity of the motor 12, the inverter 42 and/orthe capacitor 36. When braking in either the voltage-based control modeor the current-based control mode, voltage generated on the DC bus 32 bythe motor 12 is limited by the voltage limiting circuit 48. In thecurrent-based control mode, the voltage limiting circuit 48 regulatesboth i_(qref) and the motor slip frequency to limit energy generation inthe inverter 42 and on the DC bus 32 and thus avoid damage to thosecomponents. In the voltage-based control mode, the voltage limitingcircuit 48 regulates only the slip frequency.

It will be appreciated by those skilled in the art that the brakingmechanisms 102, 104 provide enhanced braking without compromisingcontrol of the braking process. The speed of the motor 12, for example,is tracked by the speed estimator 68 such that braking is enabled todecrease motor speed to any speed, including speeds other than zero. Itmay be desirable, for example, to slow the motor speed from 5,000 rpm to3,000 rpm.

In operation, the controller 22 actuates the circuit control switch 28to shift operation of the control circuit between the voltage-basedcontrol mode and the current-based control mode. The controller 22further actuates the first brake switch 108 and the second brake switch110 to use the brake mechanisms 102, 104 when in braking mode. Operationof the motor system 10 typically begins when the actual speed of themotor is zero and some non-zero reference speed is applied to the motorcontrol system 16. The controller 22 actuates the circuit control switch28 such that the switch 28 is in the first position to place the controlcircuitry in the current-based control mode. The control circuitry 20generates a driving waveform causing the motor speed to graduallyincrease toward the motor reference speed. As the motor speed increasesthrough the normal operating region, the stator voltage increaseslinearly with the speed of the motor 12, as illustrated in FIG. 4. Ifthe reference motor speed ω_(ref) is in the normal operating region, theactual motor speed reaches the reference speed and stabilizes, and thecontrol circuit 20 continues to drive the motor 12 at the referencespeed in the current-based control mode.

If the reference motor speed ω_(ref) is in the medium speed range, thecontrol circuitry 20 causes the motor speed to increase such that thestator voltage increases to and remains near the maximum availablevoltage as the actual motor speed reaches the nominal operating speedthreshold. The motor speed continues to increase in the field weakeningregion as the torque-producing component of the stator currentincreases, the flux-producing component of the stator current decreases,and the motor torque decreases. The actual motor speed eventuallyreaches the reference speed and stabilizes, and the control circuit 20continues to drive the motor 12 at the reference speed in thecurrent-based control mode.

If the reference speed is in the high speed range of FIG. 1, thecontroller 22 continues to increase the motor speed in the current-basedcontrol mode until the estimated motor speed ω_(r) reaches the highspeed range, wherein the controller 22 drives the switch 28 from thefirst position to the second position, shifting the control circuitry 20from the current-based control mode to the voltage-based control mode.In the voltage-based control mode, the control circuitry 20 operates themotor 12 at the maximum rated voltage, as explained above. As long asthe reference motor speed remains in the high speed range, the controlcircuitry 20 continues to operate in the voltage-based control mode.

While the motor 12 is operating in the high speed range, if thereference motor speed decreases the control circuitry 20 adjustsi_(qref) and i_(dref) causing the motor to operate at a negative slipfrequency until ω_(r) decreases to correspond to ω_(ref). Operating at anegative slip frequency uses the driving waveform against the motor'smovement and therefore acts as a brake. In this braking mode, themotor's movement generates electrical energy that may drive up thevoltage in the inverter 42 and/or the DC bus 32. If the voltage in theinverter 42 and/or the DC bus 32 approaches the maximum voltageV_(dcmax), the voltage limiting circuit 48 responds by limiting thenegative slip frequency to thereby limit the V_(dc) value and preventdamaging overvoltage.

If the control circuitry 20 includes the voltage-based braking mechanism104, as soon as the reference motor speed decreases to a level below theestimated speed ω_(r), the controller 22 drives the second brake switch110 to combine the voltage value V_(rated) with V_(qref) and therebyincrease braking power. When the actual motor speed approaches orreaches the reference motor speed, the controller 22 drives the secondbrake switch 110 to disengage the braking mechanism 104 from the controlcircuitry 20.

As long as the reference speed is within the high speed range, thecontrol circuitry 20 continues to operate in the voltage-based controlmode. If the reference speed decreases from the high speed range to themiddle or low speed range, however, the controller 22 switches operationof the control circuitry 20 from the voltage-based control mode to thecurrent-based control mode by driving the switch 28 from the secondposition to the first position. If the motor system 10 brakes while inthe current-based control mode, the controller 22 drives the first brakeswitch 108 to engage the current-based braking mechanism 102 asexplained above, thereby augmenting the i_(dref) signal and increasingbraking power.

The advantages of the present invention will be apparent to thoseskilled in the art. The design of the motor system 10 minimizes costs inthat it does not require the use of a braking resistor, a regenerativefront-end diode bridge, or motor shaft speed sensors. The system 10operates at maximum efficiency by operating according to a targetcurrent at lower speeds where the current margin governs operation, andoperating according to a target voltage at higher speeds where thevoltage margin governs operation. Furthermore, the motor system 10includes an enhanced braking system that can quickly brake the motor toany speed without losing the ability to measure the motor's speed.

Although the invention has been described with reference to thepreferred embodiment illustrated in the attached drawing figures, it isnoted that equivalents may be employed and substitutions made hereinwithout departing from the scope of the invention as recited in theclaims. The motor control system may be powered by a direct currentsource, for example, rather than the alternating current source asillustrated, wherein the rectifier may be omitted. Furthermore, thestator current and voltage values used in the control loops may bederived directly from motor input conductors rather than from the DCbus.

Having thus described the preferred embodiment of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:

1. A motor control system comprising: a power inverter; controlcircuitry configured to control the power inverter either according to atarget voltage in a voltage-based control mode or according to a targetcurrent in a current-based control mode; and a controller for switchingoperation of the control circuitry between the voltage-based controlmode and the current-based control mode.
 2. The motor control system ofclaim 1, further comprising a non-regenerative rectifier for receivingan alternating-current power waveform and generating direct-currentpower that is received by the power inverter.
 3. The motor controlsystem of claim 2, further comprising a speed estimator for estimating amotor speed based on at least one characteristic of the direct-currentpower generated by the rectifier.
 4. The motor control system of claim3, the speed estimator being the only source of motor speed informationused by the motor control system.
 5. The motor control system of claim3, the control circuitry including— a first regulator for generating areference current signal according to a magnitude of a target motorspeed and/or a difference between the target motor speed and anestimated motor speed, the estimated motor speed being generated by thespeed estimator; and a second regulator for adjusting the referencecurrent signal according to a difference between a measured current anda value of the reference current signal.
 6. The motor control system ofclaim 5, the first regulator and the second regulator each including aproportional-integral controller.
 7. The motor control system of claim1, further comprising circuitry for receiving first reference voltagevalues according to a rotating reference frame, converting the firstreference voltage values to second reference voltage values according toa stationary reference frame, and communicating the second referencevoltage values to the power inverter.
 8. The motor control system ofclaim 1, the control circuitry including circuit components forperforming indirect field oriented control.
 9. The motor control systemof claim 1, the controller configured to switch operation of the motorcontrol system from the current-based control mode to the voltage-basedcontrol mode when the motor control system is operating near a maximumvoltage level.
 10. The motor control system of claim 9, the controllerconfigured to switch operation of the motor control system from thecurrent-based control mode to the voltage-based control mode in a fieldweakening range of motor operation.
 11. The motor control system ofclaim 1, further comprising a frequency generation circuit fordetermining a frequency associated with a motor drive signal.
 12. Amethod of operating a motor control system, the method comprising:generating a first motor drive signal according to a target current if areference motor speed is within a first speed range; generating a secondmotor drive signal according to a target voltage if the reference motorspeed is within a second speed range; and automatically switchingoperation of the motor control system between generating the first motordrive signal and generating the second motor drive signal as the targetspeed changes between the first speed range and the second speed range.13. The method of claim 12, further comprising— using a non-regenerativerectifier to receive an alternating-current power waveform and togenerate direct-current power, communicating the direct-current power toa power inverter, and controlling the power inverter with either thefirst motor drive signal or the second motor drive signal.
 14. Themethod of claim 13, further comprising estimating a motor speed using atleast one electrical characteristic of the direct-current power.
 15. Themethod of claim 14, the step of generating the first motor drive signalincluding— adjusting a value of a reference current signal according toa target motor speed and/or a difference between the target motor speedand the estimated motor speed, and adjusting the value of the referencecurrent signal according to a difference between an estimated currentand a target current.
 16. The method of claim 15, the step of generatingthe first motor drive signal including— increasing the value of thereference current signal if the target motor speed is greater than theestimated motor speed and decreasing the value of the reference currentsignal if the target motor speed is less than the estimate motor speed,and increasing the value of the reference current signal if the targetcurrent is greater than the estimated current and decreasing the valueof the reference current signal if the target current is less than theestimated current.
 17. The method of claim 12, further comprisingreceiving first reference voltage values according to a rotationreference frame from either of the first or second control circuits,converting the first reference voltage values to second referencevoltage values according to a rotating reference frame, andcommunicating the second reference voltage values to the power inverter.18. The method of claim 12, the step of generating the first motor drivesignal including performing indirect field oriented control.
 19. Themethod of claim 12, further comprising switching operation of the motorcontrol system between generating the first motor drive signal andgenerating the second motor drive signal when the motor control systemis operating near a maximum voltage level.
 20. The method of claim 12,further comprising switching operation of the motor control systembetween generating the first motor drive signal and generating thesecond motor drive signal when the motor is operating in a fieldweakening range.
 21. A motor control system comprising: a powerinverter; control circuitry configured to control the power invertereither according to a target voltage in a voltage-based control mode oraccording to a target current in a current-based control mode, thecontrol circuitry including— a first regulator for receiving adifferential speed signal and generating a torque producing componenti_(qref) of a reference current signal based on the differential speedsignal, the differential speed signal representing a difference betweena reference motor speed and an estimated motor speed, a second regulatorfor receiving a reference speed signal and generating a flux producingcomponent i_(dref) of the reference current signal; a third regulatorfor receiving a differential i_(q) signal and generating a torqueproducing component V_(g) of a reference voltage signal, thedifferential i_(q) signal representing a difference between i_(qref) anda measured actual torque producing component i_(q) of a motor drivingwaveform; a fourth regulator for receiving a differential i_(d) signaland generating a flux producing component V_(d) of the reference voltagesignal based on the differential i_(d) signal, the differential i_(d)signal representing a difference between i_(dref) and a measured actualflux producing component i_(d) of the motor driving waveform; and acontroller for switching operation of the motor control system betweenthe voltage-based control circuit and the current-based control circuit.22. The motor control system of claim 21, the control circuitryincluding an input for receiving a maximum voltage value associated withthe motor when operating in the voltage-based control mode.